Paleointensity determination of Late Cretaceous basalts in northwest South Korea: implications for low and stable paleofield strength in the Late Cretaceous

Abstract

To clarify geomagnetic field behavior in the Late Cretaceous, paleointensity and rock-magnetic studies were carried out on basalts with K-Ar ages of 78.6 ± 2.5 and 73.1 ± 1.6 Ma in northwest South Korea. A total of 314 samples (262 from six lava flows and 52 from massive basalts) were subjected to Thellier-type IZZI paleointensity experiments. Through the application of seven paleointensity selection criteria that mainly verify the thermal alteration of magnetic minerals and the stability of remanence direction, fifteen paleointensity data were obtained from five of the lava flows. Based on rock-magnetic experiments and microscopic observations, it is revealed that the measured paleointensity is carried by single-domain (titano)magnetite. The site-mean paleointensities ranged 13.1-21.3 μT, which corresponds to a virtual axial dipole moment of 2.3-3.8 × 10 22 Am 2 . The combination of this result with selected data from the IAGA paleointensity database establishes the existence of different dipole moments according to rock type. In particular, the geomagnetic field strength recorded in Late Cretaceous crystalline volcanic rock was relatively low and stable, with a mean dipole moment of 4.0 ± 1.9 × 10 22 Am 2 regardless of geomagnetic field reversals.

Paleointensity of the geomagnetic field in the Late Cretaceous and earliest Paleogene obtained from drill cores of the Louisville seamount trail

“…Our data suggest that paleointensity of this time interval is ordinary and similar to the long‐term average intensity. Another new VADM value, 3.1 ± 0.7 × 10 22 Am 2 , has recently been obtained in this time interval by the IZZI Thellier protocol from volcanic rocks in South Korea of 73.1 ± 1.6 Ma in age [ Chang et al ., ]. This result is consistent with ours.…”

“… Comparison of paleointensity in virtual axial dipole moment (VADM) obtained in this study (red squares), that of Chang et al . [] (black square), those with the Tsunakawa‐Shaw method (orange crosses), and other available data during the last 200 Myr (PINT13) [ Tauxe and Yamazaki , ]; blue squares are from submarine basalt glasses (SBG), green circles are from single silicate crystals, and gray crosses are from whole rocks. Solid line represents the long‐term average for the last 200 Myr [ Tauxe et al ., ], and dashed line indicates the present field intensity.…”

Low geomagnetic field strength during End-Cretaceous Deccan volcanism and whole mantle convection

“…Not all volcanic rocks of PI investigation are constrained for their ages by radiometric methods. Also, some of the PI determinations come from submarine basaltic glass in which the magnetic minerals are sometimes chemically altered during PI experiments, and thus highlight the need to obtain the data from magnetically stable volcanic rocks to clarify the nature of geomagnetic field in the Late Cretaceous 21,28 . Furthermore, most submarine basalt glass values represent averages of data from individual samples and not the averages of independent time units constrained by lava flow stratigraphy and paleomagnetic directional analysis 23,28 .…”